1. Field of the Invention
The present invention relates to an image data processing system for use in image processing equipment such as a facsimile apparatus, printer, copier, etc., for the purpose of converting multi-level image data to processed image data such as binary data. In particular, the invention relates to a system wherein the image data are judged as expressing either text data or other image data such as halftone data, and selecting the type of image processing in accordance with the judgement results.
2. Description of the Related Art
There are some cases in which it is necessary to execute respectively different types of image processing in accordance with the contents expressed by an input image Signal, i.e. an input image data stream consisting of successive multi-level picture element values. In the case of a facsimile apparatus which produces binary data to be transmitted to a remote apparatus, a document page is scanned to obtain successive multi-level picture element values. The picture element values which correspond to text regions of the document page are compared with a threshold value, and converted directly to corresponding binary data. However, picture element values corresponding to halftone regions may be subjected to pseudo-halftone image processing, such as dither processing, to obtain binary data which will produce a pseudo-halftone image when subsequently printed. In such a case, it is necessary to have a capability for discriminating between text data and halftone data contained in the image data that is obtained by scanning the original document page, with such a function being referred to in the following as image region judgement. Various methods which have been proposed in the prior art for executing image region judgement include a method based on a power of a specific spatial frequency contained in the input image signal, a method utilizing differences in density between adjacent picture elements, a method based on differences between numbers of transitions between the black and white levels within a specific region, a method based on a fixed area density hystogram, etc.
As a specific example of such prior art methods of image region judgement, an image data processing system will be described which uses the power of a specific spatial frequency contained in the input image signal, referring to FIGS. 1, 2. In FIG. 1, an input image signal is supplied to an input terminal 1. The power of a specific spatial frequency within the input image signal is derived by a Fourier transform section 20, and supplied to a text/halftone discrimination section 21, which judges (i.e. for an object picture element value) whether the magnitude of the power derived by the text/halftone discrimination section 21 indicates that the object picture element value corresponds to a text region or to a halftone region of the input image. A pseudo-halftone processing section 4 executes pseudo-halftone processing of the input image data using a method such as dither processing. A bi-level conversion section 6 executes bi-level conversion processing of the input image data, and the respective processed image data outputs from the pseudo-halftone processing section 4 and bi-level conversion section 6 are supplied to input terminals designated A and B of a data selector 5, which selects one of these two types of processed image data in accordance with a decision signal that is supplied to an input terminal S thereof, to be transferred to an output terminal 7. That is to say, the type of processed image data is selected in accordance with whether it has been judged that the image data currently expressed by the input image signal are from a text region or a halftone region of an original image such as a document page.
FIG. 2 is a simple table showing the relationships between the two states of the selection signal from the text/halftone discrimination section 21 which is supplied to input terminal S of the data selector 5, and the corresponding inputs which are selected by the data selector 5.
However with such a prior art type of image data processing system, various problems arise in practice. A first problem is that it is necessary for a human operator to decide on the threshold value of spatial frequency (that frequency being referred to in the following as the reference frequency) which is to be utilized for discriminating between text data and halftone data contents of the input image signal, based on the output obtained from the Fourier transform section 20. It is difficult for a human operator to decide on a suitable value for the reference frequency. That is due to the fact that the appropriate value of the reference frequency will vary in accordance with various conditions which affect the conversion of the multi-level input image data (e.g. derived by scanning a document page) to binary data. The effects of such varying conditions will be referred to as the diversity of the input image data. For example, the operation may be affected by actions performed by a user, such as executing shading correction, variation of the threshold value that is used for converting text image data to binary data, etc. Furthermore, assuming that the input image data has been generated by scanning a document page using an optical scanner, then variations in the resultant input image data produced from the scanner can result from changes in the degree of tilt of the document sheet while being scanned, variations in the operating temperature of the image processing system, the effects of dust within the scanner, etc. Moreover if the scanned image contains pseudo-halftone contents (in addition to text regions and true halftone regions) then that will further increase the degree of diversity of the input image data. It is necessary for an image data processing system which handles such input image data to be designed such as to be capable of executing appropriate image processing irrespective of such diversity of the input image data. Hence, the design of such an image data processing system becomes extremely difficult.
Furthermore, another problem is that the diversity of the input image data is caused by a variety of different factors, so that it is necessary for a human operator to perform a large number of tests and adjustments, in order to determine a suitable value for the reference frequency. In addition, the design of the image data processing system becomes complex, due to the need to provide capabilities for counteracting all of the various effects which result in the diversity of the input image data.
In addition, if the text/halftone discrimination section utilizes the Fourier transform method, as in FIG. 1, so that a decision is made as to whether the input image data is currently representing a halftone region or a text region of the input image based on the magnitude of the power of the reference frequency that is obtained by the Fourier transform section, then if the reference frequency value is not appropriate, accurate discrimination cannot be achieved. To attempt to ensure that accurate discrimination between text and halftone data can be achieved, it has been proposed to provide a plurality of reference frequencies. However due to the fact that the algorithm becomes complicated, designing the image data processing system becomes even more difficult, and the circuit scale becomes large.
There is therefore a requirement for an image data processing system which can cope with considerable diversity of the input image data supplied thereto, but which can avoid the above problems of the prior art.